Quantitative dosimetric verification of an IMRT planning and delivery system

Background and purpose: The accuracy of dose calculation and delivery of a commercial serial tomotherapy treatment planning and delivery system (Peaco ck, NOMOS Corporation) was experimentally determined. Materials and methods: External beam fluence distributions were optimized a nd delivered to test treatment plan target volumes, including three with cy lindrical targets with diameters ranging from 2.0 to 6.2 cm and lengths of 0.9 through 4.8 cm, one using three cylindrical targets and two using C-sha ped targets surrounding a critical structure, each with different dose dist ribution optimization criteria. Computer overlays of film-measured and calc ulated planar dose distributions were used to assess the dose calculation a nd delivery spatial accuracy. A 0.125 cm(3) ionization chamber was used to conduct absolute point dosimetry verification. Thermoluminescent dosimetry chips, a small-volume ionization chamber and radiochromic film were used as independent checks of the ion chamber measurements. Results: Spatial localization accuracy was found to be better than +/-2.0 m m in the transverse axes (with one exception of 3.0 mm) and +/-1.5 mm in th e longitudinal axis. Dosimetric verification using single slice delivery ve rsions of the plans showed that the relative dose distribution was accurate to +/-2% within and outside the target volumes (in high dose and low dose gradient regions) with a mean and standard deviation for all points of -0.0 5% and 1.1%, respectively. The absolute dose per monitor unit was found to vary by +/-3.5% of the mean value due to the lack of consideration for leak age radiation and the limited scattered radiation integration in the dose c alculation algorithm. To deliver the prescribed dose, adjustment of the mon itor units by the measured ratio would be required. Conclusions: The treatment planning and delivery system offered suitably ac curate spatial registration and dose delivery of serial tomotherapy generat ed dose distributions. The quantitative dose comparisons were made as far a s possible from abutment regions and examination of the dosimetry of these regions will also be important. Because of the variability in the dose per monitor unit and the complex nature of the calculation and delivery of seri al tomotherapy, patient-specific quality assurance procedures will include a measurement of the delivered target dose. (C) 1999 Elsevier Science Irela nd Ltd. All rights reserved.